In general, systems of the prior art for measuring and controlling the color of a moving web operate by measuring the tristimulus color coordinates X, Y and Z of light reflected from a moving portion of the web. One such system is described in De Remigis U.S. Pat. No. 3,936,189 in which multiple detectors using specially designed integrating filters simultaneously measure the tristimulus values, brightness and opacity.
In my copending application Ser. No. 240,171 filed Mar. 3, 1981, now U.S. Pat. No. 4,439,038 I disclose a color spectrometer system in which a single detector is used in conjunction with a circular variable bandpass filter to yield the tristimulus values. The filter is interposed in the optical path between the web and a detector and is rotated to produce a detector output which periodically scans the optical spectrum. While this system is generally satisfactory, it does not permit a fast scan color, brightness and opacity measurement as it uses only one detector which provides output related to only a small portion of the width of the moving web.